The present invention relates generally to spinal surgery and, more particularly, to improved spinal fixation devices and methods of using the same.
The human spine exhibits some degree of curvature at different levels to facilitate normal physiologic function. Correction may be required when this curvature deviates substantially. A common problem is lateral deviation of the spine, commonly termed scoliosis.
As discussed in U.S. Pat. No. 5,540,689, the first successful internal fixation method for surgically treating scoliosis used the Harrington instrumentation system. According to this technique, a rigid rod with hooks at each end is implanted adjacent the concave side of the scoliotic spine. The spine is manually straightened to a desired extent and a distraction rod is used to maintain the correction by exerting vertical forces at each end. The rod commonly has a ratcheted end over which hooks are slidably mounted and locked in place. To accommodate lordosis, a compression rod is sometimes placed on the convex side of the scoliotic spine.
The Harrington instrumentation system has been used successfully for some time, but because the distraction rod is fixed to the spine in only two places, failure at either end causes the entire system to fail. An alternative treatment has since evolved which takes advantage of segmented fixation. In this method, a rod is fixed to the spine at multiple points by means of a sublaminar wires which run underneath the lamina of the vertebra and around the rod. The use of multiple fixation sites enhances stability and reduces the need for additional post-operative bracing.
Sublaminar fixation utilizing current devices has two primary weaknesses, however. First, the wires are simply wrapped around the rod, resulting in a rod to cable junction which is not rigid. Second, the thin wires can cut in some instances right through the lamina. Thus, the need remains for a more structured approach to this procedure.
This invention solves problems associated with existing sublaminar fixation techniques by providing devices, instrumentation, and surgical techniques for a more structured coupling of lamina or other bony spinal structures to an alignment rod.
In terms of apparatus, the invention provides a body having an aperture formed therethrough to receive the rod, a mechanism to lock the body in place once a desired position is established along the rod, and two or more holes through the body to receive a cable wrapped around at least a portion of each vertebra to be stabilized. A sleeve may also be provided over the cable where it wraps around the structure to distribute the force of the cable against the bone to prevent damage.
In a preferred embodiment, the mechanism to lock the body in place includes a threaded fastener having an exposed proximal end for tightening and a distal end that bears against the rod. The body may be unitary, or provided in sections, one with the aperture and the other with the cable receiving holes, and with the two sections being rotatable relative to one another. The orientation of the cable-receiving holes may be varied relative to the aperture for the rod for convenient cable connection to different spinal structures such as the lamina and/or spinous process. At least one of the cable-receiving holes may also be flared or ring-shaped to facilitate a range of motion to avoid cable fatigue.
The invention also provides instrumentation facilitating sublaminar cable fixation, comprising a tool having a proximal end with a handle and a distal end. The distal end terminates in a curved, cannulated section between two cable-receiving holes, the length of the curved section and the distance between the holes being such that the proximal end may be positioned under a lamina or other bony structure to feed the cable therearound.
FIG. 1 is a perspective view of a rod-mounted cable holding sublaminar fixation device according to the invention;
FIG. 2A is the first drawing of a sequence, showing how cables may be progressively tensioned using the inventions;
FIG. 2B shows a subsequent step relative to FIG. 2A, wherein a lamina is pulled closer to a inventive device and recrimped, in this case;
FIG. 3A is a drawing which shows a device according to the invention having an alternative cable holder, wherein a cup-shape recess or ring is used in conjunction with a ball-shaped cable end for a greater range of motion;
FIG. 3B is a drawing which illustrates the improved range of motion made possible by the device according to FIG. 3A;
FIG. 4 is a drawing which shows how separate rod-holding and cable-holding sections may be rotatably coupled to one another;
FIG. 5A is a drawing of a covering in the form of a resilient sleeve to distribute forces and protect laminar bone;
FIG. 5B is an alternative covering using wire mesh;
FIG. 5C is yet a further force-distributing sleeve using a corregated surface to facilitate bending;
FIG. 6 is a posterior view of a spine with scoliosis used to show how devices according to the invention may be longitudinally positioned to accommodate distraction and compression; and
FIG. 7 is a drawing of a tool according to the invention used to dress cables around and behind a lamina for use with the devices described herein.